1. Field of the Invention
The present invention relates to an optical fibre cable which comprises at least one metallic member. More particularly, it the prevention and location of breaks or cracks in one optical fibre of the cable which has been subjected to the effects of tension. Although the invention falls within the general scope of ground or submarine optical fibre cables, reference will chiefly be made hereinafter to submarine cables that experience considerable mechanical strains, due to high submarine pressures reaching 6 to 8 daN/mm.sup.2.
2. Description of the Prior Art
It is known that a submarine cable, during laying and/or raising, withstand tensile strengths having values depending, inter alia, on the ocean depth at the laying or raising location. These strengths reach a maximum in the portion of the cable nearest the surface of the ocean. When laying or raising the cable in great depths (6000 meters), the mechanical strains give rise to local elongated effects which depends on the features of the cable. The average values for these elongations are estimated at 0.3 and 1 percent respectively under conventional laying and raising conditions using a cable-ship. Furthermore, the heaving and pitching movements of a cable-ship caused by ocean swell bring about transient increases in the cable tension. Calculations show that additional transient elongations of upto 0.2% are to be expected when laying or raising the cable in heavy weather conditions.
As a result, an underwater optical fibre cable must withstand elongations of greater than 1% without breaking or embrittling of the optical fibres it contains.
The known structures of underwater optical fibre cables are, moreover, designed to minimize any elongations and hence stresses sustained by the optical fibres when the cable is stretched.
It may be added that fibre lengths of upto a few kilometers can now be manufactured that display relatively high mechanical features, capable of withstanding elongations of some 5%.
The problem, nonetheless, still exists of industrially producing very long lengths of fibres capable of withstanding considerable elongations with no risk or breakage. The on-going debugging operations, for instance, on these long lengths of fibre, intended for testing the fibre tensile strength, could also quite well produce undesirable embrittlement of the fibre by crack growth (without breakage) in the course of the test.
The possibility of a break in the fibres occuring during a laying or raising operation cannot therefore be excluded.
There may well also be embrittlement of the fibres--crack grouwth without breakage--due to excessive local elongation in the cable during these operations. As the fibres in a cable are always undergoing some strain, the static fatigue phenomenon--long term fibre breakage at low strain as a result of slow crack growth--could come about in those sections where the cable has been overstretched thus leading to subsequent fibre breakage in the laid cable.
In the event of a breakdown in the transmission of optical signals conveyed by a cable optical fibre, following a break in a fibre without breakage of the cable, means must be available for locating the position where the break has occured with a view to raising and repairing the corresponding portion of the cable.
When the cable is short and does not require repeaters, the break in an optical fibre can be located by a back-diffusion procedure, as set forth in U.S. Pat. No. 4,012,149. A light impulse is supplied the fibre through an accessible end of the cable, such as a terminal station, and is reflected by the break back to that station. From the propagation time measurement between emission and reception of the light impulse, the distance between the break and the terminal station is deduced.
On the other hand, when the cable is long and includes repeaters, an optical remote-location system for finding fibre breaks would require repeater structures of a complexity such that there would be no advantage to be gained in comparison with the advantages set forth here. Back-diffusion means would have to be introduced into each repeater that would be capable of testing each optical fibre from the repeater and the information obtained would then be transmitted to the terminal station.